A conventional drill chuck has a chuck body which defines and is rotatable about a chuck axis, and which carries a plurality of generally angularly equispaced and radially displaceable jaws. A tightening ring, normally linked to a tightening sleeve, is rotatable on the chuck body about the axis. Formations between the jaws, the tightening ring, and the body serve to radially displace the jaws on the body on rotation of the ring on the body about the chuck axis. These formations normally include interengaging screw threads between the jaws and the tightening ring and inclined guides in the body supporting the jaws so that same move axially as well as radially when tightened or loosened. It is also standard practice to tighten such a chuck by means of a key. To this end the ring is formed with an array of axially directed teeth centered on the chuck axis. The chuck body is formed with at least one radially outwardly open hole adjacent this array of teeth. A key is formed with a cylindrical projection engageable in this hole, and with a gear that meshes with the teeth when the projection is engaged in the recess. Thus rotation of the key while its projection is engaged in the chuck-body recess will rotate the tightening ring on the chuck body and thereby tighten or loosen the jaws. Obviously a transverse handle on this key allows considerable torque to be brought to bear, so that the chuck can be drawn very tight, or loosened once it is very tight.
The principal disadvantage of this prior-art arrangement is that at least some of the forces inherently effective on the jaws during drilling, and in particular during hammer drilling when the chuck is reciprocated axially as well as being rotated about its axis, have a component which is effective on the tightening ring in the direction tending to loosen the chuck. Thus under some circumstances the drill chuck will loosen.
Accordingly it is known, as for example from the above-cited copending applications as well as from, for instance, German published patent specification No. 2,639,214, to provide a system for positively locking the tightening ring in the chuck body. Such a system normally comprises a multiplicity of angularly equispaced teeth or stops formed on the tightening ring and engageable by one or more members affixed on the chuck body.
Such an arrangement has two principal disadvantages. First of all no matter how many such stops or teeth are provided, the tightening ring can only be locked in a finite number of angularly offset positions relative to the chuck body. For this reason some loosening is normally possible unless it so happens that the chuck is tightened to a position exactly corresponding to engagement of the locking element with one of the stops or teeth. Furthermore when such an arrangement is used in a hammer drill it is possible that the violent forces to which the various components of the chuck are subjected will allow the locking member to move momentarily out of contact with its stop, so that the tightening ring can loosen in spite of the provision of the special locking arrangement.